Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching similar to those developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors.
Digital micromirror devices have been utilized in optical display systems. In these display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of many digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane that, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Thin hinge structures, which restrict the deformation to a relatively small region of the device, limit the amount of light scattered and improve image quality.
Torsion beam devices enabled the use of dark field optical systems. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface. The rigid member or mirror is suspended by, and typically centered on, the torsion hinge-allowing the mirror to rotate by twisting the torsion hinge. Address electrodes are formed on the substrate beneath the mirror on either side of the torsion hinge axis. Electrostatic attraction between an address electrode and the mirror, which in effect form the two plates of an air gap capacitor, is used to rotate the mirror about the longitudinal axis of the hinge.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by using an elevated mirror to block most of the light from reaching the torsion beam hinges. The elevated mirror is connected to an underlying torsion beam or yoke by a support post. The yoke is attached to the torsion hinges, which in turn are connected to rigid support posts. Because the structures that support the mirror and allow it to rotate are underneath the mirror instead of around the perimeter of the mirror, virtually the entire surface of the device is used to fabricate the mirror. Since virtually all of the light incident on a hidden-hinge micromirror device reaches an active mirror surface-and is thus either used to form an image pixel or is reflected away from the image to a light trap-the hidden-hinge device's contrast ratio is much higher than the contrast ratio of previous devices.
Images are created by positioning the DMD so that a light beam strikes the DMD at an angle equal to twice the angle of rotation. In this position, the mirrors fully rotated toward the light source reflect light in a direction normal to the surface of the micromirror device and into the aperture of a projection lens-transmitting light to a pixel on the image plane. Mirrors rotated away from the light source reflect light away from the projection lens-leaving the corresponding pixel dark.
Full-color images are generated either by using three micromirror devices to produce three single-color images, or by sequentially forming three single-color images using a single micromirror device illuminated by a beam of light passing through three color filters mounted on a rotating color wheel.
An example of a small portion of a digital micromirror array is depicted in FIG. 1. In FIG. 1, a small portion of a digital micromirror array 100 with several mirrors 102 is depicted. Some of the mirrors 102 have been removed to show the underlying structure of the DMD array. FIG. 2 is an exploded close-up of one individual mirror 102 of a DMD array. The electrical interconnections and operations of the individual micromirrors 102 are described in further detail in U.S. Pat. No. 6,323,982 entitled Yield Superstructure for Digital Micromirror Device to Larry J. Hornbeck, which is hereby incorporated by reference.
A representative example of an existing spatial light modulator (SLM) device 300 is depicted in FIG. 3. In FIG. 3, a micromirror array 100 is mounted onto a ceramic base 305, which is further mounted onto a printed circuit board (PCB) or electronic lead package 310. The micromirror array 100 is electrically connected to the ceramic base 3 05 by a series of leads 315. Each of the leads 315 is connected to the micromirror array 100 at bonding pads 320 that are integral to the micromirror array 100. At the other end, the leads 315 are connected to bonding pads 325, which are integral to the ceramic base 305. Each of the bonding pads 325 on the ceramic base 305 is connected to a series of land grid array (LGA) pads on the bottom of the ceramic base 305 through internal interconnect layers that are built into the ceramic base. The LGA pads may then be connected to the PCB 310 via elastomer or C-spring connectors. In order to ensure a reliable electrical connection between the ceramic base 305 and the PCB 310, a mechanical loading must be applied between the ceramic base 305 and the PCB 310. FIG. 3 describes only one representative embodiment for electrically connecting the micromirror array 100. Other suitable arrangements for electrically connecting the micromirror array 100 are well known in the art.
The micromirror array 100 depicted in FIG. 3 is hermetically sealed in the SLM device 300 to prevent the array 100 from becoming damaged. To accomplish this, a seal ring 335 is disposed on the ceramic base 305 so that the micromirror array 100 is surrounded. A window frame 340, which incorporates an optically transparent piece 345, is mounted onto the seal ring 335 to form a seal that encases the micromirror array 100. The window frame 340 comprises a single block of metal or other suitable material that has been formed through forging or CNC machining. The transparent piece 345 is typically a piece of glass, or other optically transmissive material, that is mounted and sealed into the window frame 340. The window frame 340 is mounted onto the seal ring 335 by seam welding or other suitable processes that forms a seal between the seal ring 335 and the frame 340.
A cross-sectional view of the SLM device 300 depicted in FIG. 3 along the axis I-I is depicted in FIG. 4. Many of the same components described above (the micromirror array 100, the ceramic base 305, the electronic lead package 310, the electronic leads 315, bonding pads 320 and 325, the seal ring 335, the window frame 340, and the transparent piece 345) are depicted in FIG. 4. Also depicted is a heat sink 400 that is mounted onto the bottom of the base 305. The heat sink 400 absorbs the heat generated by the micromirror array 100 when it is illuminated with a light source during its operation. Further details of the components and connections of a typical SLM device are described in U.S. Pat. No. 5,936,758 entitled Method of Passivating a Micromechanical Device Within a Hermetic Package to Edward C. Fisher, et al., which is hereby incorporated by reference.
A problem associated with existing SLM devices 300 occurs as the devices are subjected to thermal and mechanical stresses during the mounting and operating processes. For example, as the window frame 340 is seam welded onto the seal ring 335, a great deal of thermal and mechanical stresses are applied to the window frame 340. These stresses can weaken or break the seal between the window piece 345 and the window frame 340. Furthermore, as the SLP device 300 is mounted onto a printed circuit board 310 by applying a mechanical bias between the ceramic base 305 and the PCB 310, mechanical stresses are transferred to the seam weld between the seal ring 335 and the window frame 340 and to the seal between the window frame 340 and the window piece 345. In addition, as the SLM device 300 is illuminated and operated, a large amount of heat is generated by the DMD array 100, the window frame 340, and the window piece 345. This is particularly problematic when the coefficient of thermal expansion of the various components is not matched. This heat can induce mechanical stress on the seams and welds mentioned above. Accordingly, there is a need in the art for a window frame and window piece assembly that can relieve the mechanical stresses that arise during the mounting and operating processes of the SLM device 300.